The present invention relates to a die device for a blow-moulding apparatus for the production of a multi layered blown film, a blow-moulding apparatus for the production of a multi layered blown film and a method for temperature controlling a multi layered material melt during the production of a multi layered blown film.
It is known that for the production of blown films blow-moulding apparatus are used. Thereby, commonly a material melt is dispensed through a die orifice and subsequently cooled down in blowing form. The cooling commonly occurs from the outer surface and/or the inner surface through an air cooling device. In this manner not only single layered but also multi layered blown films can be provided. With the production of multi layered blown films die orifices are configured for the outlet of a multi layered material melt. This means that through the die orifice different materials in the melt are dispended layerwise towards one another which are already merged within the blown head comprising the die orifice. The document EP 1 055 504 discloses such a blown head. Thereby, the corresponding layers of the material melt correlate with the corresponding layers subsequently to the cooling down, namely the layers of the blown film. The multi layered blown film will correspondingly comprise an inner surface and an outer surface between which the layers of the multi layered blown film extend. Accordingly, the material melt comprises two sides which correspond with the subsequently produced inner surface and outer surface of the blown film and include the layers according to the material melt.
With known die devices or with known blow-moulding apparatus it has to be proceed with great accuracy during the production of multi layered blown films. During the outlet from the die orifice the multi layered material melt will comprise a temperature in which all materials of all layers are available in a melted state. Since however normally each layer comprises a different material type the different layers also comprise different physical properties. Thereby a particularly important physical property is the viscosity. Thereby, each material, which comprises a different temperature viscosity curve, will also comprise a different viscosity during the melting temperature at the outlet through the die orifice. With other words, at the point of time of the outlet from the die orifice a distribution of the different viscosities exists over the width of the material melt. This viscosity distribution leads to different flexibilities of the single layers, so that by the conveyance away from the die orifice a deflection of the leaked material melt to the inside, meaning in direction of the to be performed inner surface, or to the outside, meaning in the direction of the to be built outer surface, occurs. This concavity to the inside or to the outside produces additional tension within the film. Thereby, by viscosity differences this deflection can be as great that a contact between the escaping material melt and parts of the die device particularly at the edge of the die orifice or the cooling devices occurs. Such a contact necessarily leads to the adhesion of the material melt at this position. An adhesion however conflicts with the continuous outlet of the material melt so that at this position a hole or even a crack of the material melt occurs. The subsequently cooled down material melt or the thereby developed blown film will accordingly rip off, so that a stop of the complete manufactory process becomes necessary. This leads to a material waste and to cost intensive operating life.